Technical Solutions for the Implementation of a PBI VMAT Technique at BC Cancer Vancouver
Abstract
Purpose
A partial breast irradiation (PBI) VMAT technique has been implemented at our center based on clinical trial evidence that showed reduced long-term toxicity when compared with whole breast irradiation. Our use case for PBI includes difficult anatomy like very large breasts or the adjacency of an organ at risk to the target, and the need for delivering a treatment with an integrated rather than sequential boost. We present our solutions to the challenges of planning and treating a mobile, lateral target with a highly conformal technique.
Methods
For ease and consistency of target delineation and image guidance on treatment, guidelines were provided to breast surgeons on the placement of surgical clips at the site of the tumor bed. For image matching purposes, clips were contoured with additional margins matching those of the PTV. A new CBCT technique was commissioned with a shortened trajectory to ease breath-hold CBCTs. To ensure safe image acquisition for very lateral targets within the mechanical limits of a C-arm linear accelerator, a look-up table with allowable isocenter locations was created to ensure imaging arcs would not cause a collision. Such isocenters often meant that the target was off-axis during treatment, so jaw tracking was commissioned to reduce low dose spill outside of the target.
Results
PBI was successfully implemented with the targeted solutions as outlined. A planning workflow was introduced that included imaging clearance based isocenter selection. CBCT IGRT based on matching surgical clips was highly effective and allowed for fast patient treatment with reduced matching ambiguity.
Conclusion
The Breast Technical Group at our center effectively introduced PBI as an alternative to tangent irradiation.